PPC - Predictive Process Control
PPC is the form of process control in which:
o
one knows (through measurements) the properties and characteristics of
the raw materials
that will be used in the next production batch, and
o
one uses that knowledge to predictively control to achieve desired batch
properties.
Many processing plants
use constant recipes for all batches, relying on the suppliers' abilities
to deliver constant property, in-spec materials day after day after day.
A fact of nature is that all properties of all materials always vary.
Putting these two together, we can draw the following conclusion: constant
compositions and varying raw materials properties always produce bodies
with varying properties.
But when one measures all relevant
control properties for each raw material prior to batch formulation, today's
computers can quickly calculate the compositions required to achieve constant
batch properties. Then, with a variable formulation, one can achieve constant
batch properties.
The particular properties that need to be measured and controlled using
PPC may include particle size distribution parameters, specific surface
areas, interparticle spacings, and/or oxide composition percentages, to
name but a few.
Some might think that a variable
composition would lead to a lack of control and a variety of output body
compositions, but when applied properly, production bodies actually can
be more uniform than ever before (and this has been proven to be the case
in plants that are running under PPC control.)
An example will help to demonstrate
this concept. One of the major impurities in raw ball clays is free quartz.
When ball clay is a fixed percentage of a batch, free quartz impurity
additions will also be fixed, if, and only if, the free quartz is a constant
percentage in the ball clay. If the quartz content varies, the actual
amounts of both ball clay and quartz added to the body will vary. Bodies
containing ball clays frequently also have flint (quartz) as another ingredient.
When the ball clay contains free quartz, more clay and less flint should
be added to achieve target ball clay and quartz contents in the body.
To make the numbers simple,
let's assume a body contains 25% ball clay and 25% flint. If it's known
that the ball clay contains ~4% free quartz, the body composition should
be adjusted to include enough "ball clay" to achieve the desired
25% clay, and flint additions should be decreased accordingly. In this
case, adding 26% of the "ball clay" (containing 4% free quartz)
will contribute about 25% ball clay and 1% free quartz to the batch. Since
1% quartz arrives with the ball clay, only 24% flint needs to be added
to achieve the target values of 25% ball clay and 25% quartz.
Then, for subsequent batches,
if the impurity free quartz levels in the ball clay vary from 4%, the
new composition can be adjusted to maintain the desired 25% clay and 25%
quartz contents in the body.
Simply put, such adjustments
constitute PPC control of the body. But rather than controlling only one
property, PPC monitors and controls a variety of important body properties.
Companies using fixed recipes
frequently don't make such adjustments, nor do they pay much attention
to compositional or physical property changes of the incoming raw materials.
Under PPC control, the control
parameters for each important body property are identified, measured,
and used to control body compositions. This allows the important body
properties to be controlled, on a batch to batch basis, to achieve constant
body properties.
The full details of this procedure,
known as PPC, are the main subjects of the textbook written by James Funk
and Dennis Dinger, Predictive Process Control of Crowded Particulate Suspensions
Applied to Ceramic Manufacturing, printed by Kluwer Academic Publishers
in 1994.
To learn more specific details about how PPC can be implemented in your plant, please contact us.
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